1. Field of the Invention
This invention relates in general to drive and control technology for stepping motors and particularly relates to generating a motor clock output with a variable frequency for driving stepping motors during individual operating phases.
2. Description of the Prior Art
Circuit arrangements for driving stepping motors usually comprise a clock generator, a control stage and a motor amplifier. The clock generator generates the motor clock output with the desired motor frequency. Switching pulses cyclically following one another with the motor frequency are derived in the control stage from the motor clock's output. The motor amplifier comprises a DC voltage source and switches controlled by the switching pulses and these switches are connected to the DC voltage source and to the stator windings of the stepping motor. As a result of the cyclical supplying by the switches of the motor frequency, the rotary field for the rotor of the stepping motor is generated and the speed of stepping is dependent on the motor frequency.
A stepping motor has a limiting frequency dependent upon the coupled load at which it no longer starts up without stepping errors on the basis of merely switching the motor clock input on and in which it no longer comes to a standstill with step precision by switching the motor clock output off. So as to avoid stepping errors particularly at high stepping frequencies, the stepping motor in a run-up phase having a rising motor frequency is thereby pulled up to the desired operating frequency from a low starting frequency and is subsequently stopped with step precision in a following deceleration phase by lowering the motor frequency.
The clock generator must therefore supply a motor clock output which has a frequency chronologically variable during run-up and during the decelerating phases and is constant in the work phase. So as to generate a motor clock sequence with a chronologically variable frequency a traditional clock generator is formed with, for example, frequency voltage transformers wherein the chronological frequency curve is dependent on a control voltage. The employment of frequency-to-voltage transformers, however, has the disadvantage in that the curve of the control voltage must be simulated by timing elements, for example, by charging and discharging capacitors and these are used especially when different frequency curves must be available. It is also relatively difficult to hold the control voltage constant during the work phase and to synchronize it with other control parameters during the run-up and decelerating phases.
Another type of apparatus for generating a motor clock output with a variable motor frequency is disclosed in German A No. 22 38 613 wherein the clock generator is composed of a clock generator that generates a basic clock output having a constant basic frequency and also includes a following frequency divider stage constructed of individual flipflops in which the required motor frequency is acquired from the basic frequency of the basic clock output by frequency division. The variable motor frequency is generated by means of chronologically varying the division factor for example with the assistance of timing elements.
German C No. 27 21 240 discloses another clock generator comprising a clock generator and frequency divider stage. The required division factors are prescribed therein as data words which are deposited in a read-only memory as a program sequence and are output dependent on the executed steps of the stepping motor in order to operate the stepping motor with optimum load angle.
In the known clock generators having traditionally constructed frequency divider stages, disturbing frequency discontinuities occur during step-by-step switching of the frequency divider stages and these discontinuities are especially great and disturbing when small division factors are used. There is a risk that the stepping motor will make step errors or even fall out of step as a consequence of these frequency discontinuities.
However, small division factors always occur when high motor frequencies approaching the constant basic frequency are required. Also, the torque of the stepping motor decreases with increasing motor frequency whereby the risk of step errors or of falling out of step increases when frequency discontinuities occur. For this reason, the division factors for the highest motor frequency should be as large as possible so that the disturbing frequency discontinuities remain small. In order to meet this requirement, a basic clock sequence having an extremely high basic frequency must be generated and this is realizable only with high technology devices.